Object stream example

object-stream-example.js

// Number stream example
"use strict"

const stream = require("stream")

let numberStream = new class extends stream.Readable {
    constructor () {
        // This is an object stream
        super({ objectMode: true })
        this.counter = 0
    }

    _read () {
        this.push(this.counter++)
    }
}

let squareStringStream = new class extends stream.Transform {
    constructor () {
        // This is an object stream
        super({ objectMode: true })
    }

    _transform (number, _, callback) {
        this.push(`${number * number}\n`)
        // Callback must be called to verify that write succeeded
        callback()
    }

    _final () {
    }
}

numberStream.pipe(squareStringStream)
squareStringStream.pipe(process.stdout)

// Exit process on EPIPE (stdout is closed, i.e. when piped to head(1)
// command)
process.stdout.on("error", (...stuff) => process.exit(0))

You could actually split squareStringStream into squareStream, which would square numbers, and lineStream, which would produce "\n"-postfixed strings from input objects, but that'd be boring, so I haven't done that.

Important parts are object mode flag, so the whole thing doesn't choke on trying to push something that isn't either Buffer or String:

super({ objectMode: true })

and EPIPE guard^note, which is pretty much only useful for command line programs:

process.stdout.on("error", (...stuff) => process.exit(0))

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^note famous epipe problem solved in recent Node releases is a different issue, and you still have to guard against EPIPE when manually redirecting streams to stdout